Growth and Survival of Salmonella montevideo on Tomatoes and Disinfection with Chlorinated Water

Quantification of Escherichia coli Transfer from Plastic Mulch to Field-grown Tomatoes and Bell Peppers

In the United States, the Food and Drug Administration (USFDA) prohibits the distribution of fruit that is dropped from the plant and contacts the ground during harvest. This includes fruit which contacts the ground while attached to the plant, called "drooping" fruit. In the Southeastern US, tomato and pepper are trellised and grown on plastic mulch. The objective of this study was to obtain bacterial transfer rates from a nonpathogenic Escherichia coli GFP inoculated on plastic mulch (black and white) to fruit (tomatoes and peppers) that contact the ground by being dropped (at heights of 30, 60, or 120 cm) or by drooping (contact times of 1 h or 24 h) in the field during the summer season in Georgia, USA. Plastic mulch was surface inoculated with E. coli (106 CFU/64 cm2), and after drying, populations were reduced by >2-3 log CFU/64 cm2. Once inoculum was dry, the fruit was either dropped from different heights through a PVC pipe or placed back onto the mulch in its initial resting place. The mean log percent transfer of E. coli from plastic mulch to dropped tomato and pepper fruit was -2.00 to 0.46 (0.01-2.88%). Mean log percent transfer rates of E. coli to drooping fruit were between -0.83 and 0.01 (0.15-1.02%), with no significant differences in transfer within crop types between treatments of plastic mulch color or contact time. Field environmental conditions throughout the experiment such as ambient air temperature, relative humidity, UVAB radiation intensity, and surface temperature of plastic likely affected the rates of bacterial transfer. While other studies have evaluated bacterial survival and transfer from mulch to fruit in a laboratory setting, the present study addresses knowledge gaps in bacterial transfer during drooping and dropping incidents in the field when fruit contacts plastic mulch, providing results that have potential to inform future regulatory guidance for produce harvest and handling.

Global bibliometric meta-analytic assessment of research trends on microbial chlorine resistance in drinking water/water treatment systems

Chlorine is the commonest and cheapest disinfectant used in drinking water and wastewater treatment at household, municipal and industrial levels. However, the uprising of microbial chlorine resistance (MCR) pose critical public health hazard concerns; because, its potentiate exposure to difficult-to-treat resistant pathogens. Therefore, this study aimed at evaluating the burden of MCR in drinking water/wastewater treatment and distribution systems (DWWTDS) via science mapping of research productivity (authors, countries, institutions), thematic conceptual framework, disciplines, research networks and associated intellectual landscape. MCR data were mined from Scopus and Web of Science based on optimized algorithms with the root key term "chlorine* resistant*'' and analysed for pre-set indicator variables. Results revealed 1127 documents from 442 journals and 1430% average growth rate (AGR) of research articles from 2017 to 2019 on MCR. Country-wise, the USA (n = 299), China (n = 119), and Japan (n = 43) ranked in the 1st, 2nd, and 3rd positions respectively, among the top participating countries in MCR research. MCR research had considerable performance in public health and sustainable concern subjects namely, Environmental Sciences & Ecology, Engineering, Microbiology, Water Resources, Biotechnology & Applied Microbiology, Food Science & Technology, Public, Environ & Occupational Health, Chemistry, Infectious Diseases, and Marine & Freshwater Biology; and with noticeable AGR in Environmental Sciences & Ecology (330%) and Infectious Diseases (130%). The study found biofilm-related thrusts (n = 90, 270% AGR) as main research hotspots on MCR. Overall, the study identified and discussed four important thematic areas of public health challenges in MCR that could promote increasing waterborne diseases due to (re)emerging pathogens, enteric viruses and dissemination in DWWTDS. In conclusion, this study provides comprehensive overview of the growing burden of MCR in DWWTDS and standout as a primer of information for researchers on MCR. It recommends direct, intentional and integrated research priorities on MCR to overcome accompanying public health and environmental threats. In addition, chlorine resistance in waterborne fungi have not received research attention. Research activities related to fungal chlorine resistance will be an invaluable future direction in DWWTDS and guide against exposure to waterborne pathogenic fungi and mycotoxins. It is unknown whether chlorine resistance can be acquired by horizontal gene transfer in microorganisms and future research should elucidate this important thrust.

Transfer efficacy of Escherichia coli O157:H7 between surfaces of green mature tomatoes and common food processing materials

The objectives of this study were: a) to evaluate E. coli O157:H7 survival on green mature tomatoes and squares of common food processing materials – stainless steel, plastic (HDPE), and vinyl conveyor belt (PVC) – post-drying, stored at 25 ºC in the humidified environment for four days; b) to determine pathogen transfer rates (wet, 90 minutes, or 24-hours drying post-inoculation), from inoculated tomato surfaces to uninoculated steel, plastic, and vinyl conveyor belt squares and conversely. It was shown that E. coli O157:H7 did not survive well on the surface of tomatoes, resulting in a decline from 5.3 log10 CFU.mL-1 90 minutes post-drying to 1.4 log10 CFU.mL-1 on day 4. Similarly, the pathogen did not survive well on the surface of food processing squares, with numbers declining over 4 days from 4.04, 4.44, and 4.19 CFU.mL-1 of rinsate 90 minutes squares post-drying to 0.72, 0.50, 0.83  log10 CFU.mL-1, which is close to the detection limit, for the steel, vinyl belt, and plastic, respectively. Successful cross-contamination between tomatoes and food processing surfaces was achieved during wet transfer; while transfer after 90 minutes inoculum post-drying and 24 hours were less successful. This can be explained by both lack of liquid media with suspended bacteria for transfer and fast pathogen die-off after desiccation. Dry transfers, as shown by the percentage of “positive” for pathogen presence tomatoes and squares, as well as bacterial counts, were more successful from tomatoes to squares, but not conversely. Special concern raised vinyl conveyor belt, where the surface picked up the most pathogen cells from the surface of tomatoes, resulting in 100% positive during 90 minute-dry transfers, followed by plastic (66.7% positive) and steel (55.6% positive). To summarize, we presented data on the possibility of cross-contamination between mature green tomatoes and common food processing surfaces, which may be interesting for the processors for risk evaluation.

Influence of suspension liquid total solids on E. coli O157:H7 survival and transfer efficacy between green tomatoes and cardboard

The objectives of this study were: a) to determine E. coli O157:H7 survival on tomatoes and cardboard squares post-drying, stored at 25 ºC in humidified environment for four days, in buffered peptone water (BPW), and 0.1% diluted peptone (DP); b) to determine pathogen transfer rates (0, 1.5, or 24-hours drying post-inoculation), from inoculated tomato surfaces to uninoculated cardboard squares and conversely; and c) to evaluate SystemSure Plus ATP luminometer for recognizing contamination on visibly soiled (BPW) or visible clean (DP) cardboard. In tomato inoculation studies, E. coli O157:H7 survived better on the fruit when the inoculum was prepared using DP as compared to BPW. The 1.5-hours post drying counts of 5.34 and 5.76 log10 CFU.mL-1 in the rinsate substantially declined to 1.45 and 1.17 log10 CFU.mL-1 on day four, for DP and BPW, respectively. In cardboard inoculation studies, E. coli O157:H7 persisted for four days, with 1.5-hours post-drying counts and day four counts of 4.53 (DP) and 2.55 log10 CFU.mL-1 (BPW), contrary to 3.81 (DP) and 1.92 log10 CFU.mL-1 (BPW). Under the first impression, the slower die-off of E. coli O157:H7 on cardboard questions the possibility of reusing cardboard boxes due to the potential for cross-contamination. In wet transfer (0 hour drying) trials, both tomato-to-cardboard and cardboard-to-tomato yielded 100% positive transfers irrespective of diluent type. Dry transfer (1.5-hours drying interval post inoculation) from tomato-to-cardboard were 100% positive, but no positives were noted when inoculated, dried cardboard was contacted to tomatoes, irrespective of diluent. Results of transfers with BPW as the diluent showed 100% positive transfer from 24-hours dry tomatoes-to-cardboard, as inoculation spots on the tomatoes remained moist due to hygroscopic nature of solutes in BPW. Conversely, only a 40% positive transfer rate was observed under the same conditions with DP as diluent. No positive transfers were recorded from 24-hours dry cardboard-to-tomatoes, irrespective of diluent type. Though E. coli O157:H7 survived better on the surface of cardboard compared to the surface of tomatoes on day four, the dry transfers were more efficient from tomatoes-to-cardboard than conversely, possibly due to smooth and hydrophobic properties of the tomato, and rough and porous surface of the cardboard. ATP luciferase UltrasnapTM swab test showed 9/9 “pass” results for sterile liquid DP and BPW, while 9/9 “fail” results were observed with liquid peptone and BPW contaminated at ca. 9.0 log10 CFU.mL-1E. coli O157:H7. Cardboard squares treated and dried, with sterile DP, showed 8/9 “pass” ATP luciferase results, and 1/9 “warning”, while cardboard squares with contaminated DP showed 9/9 “fail” result. Cardboard squares treated and dried, with sterile BPW, showed 7/9 “pass” ATP luciferase results, and 2/9 “warning”, while cardboard squares with contaminated BPW showed 9/9 “fail” result. Luminometer can simplify detection of microbial load, as well as organic residues, helping to check cardboard boxes for cleanness.

Sources of human infection by Salmonella enterica serotype Javiana: A systematic review

Non-typhoidal Salmonella (NTS) infection is one of the major causes of diarrheal disease throughout the world. In recent years, an increase in human S. Javiana infection has been reported from the southern part of the United States. However, the sources and routes of transmission of this Salmonella serotype are not well understood. The objective of this study was to perform a systematic review of the literature to identify risk factors for human S. Javiana infection. Using PRISMA guidelines, we conducted a systematic search in Web of Science, PubMed, and the Morbidity and Mortality Weekly Report (MMWR). Searches returned 63 potential articles, of which 12 articles met all eligibility criteria and were included in this review. A review of the literature indicated that both food and non-food (such as animal contact) exposures are responsible for the transmission of S. Javiana infection to humans. Consumption of fresh produce (tomatoes and watermelons), herbs (paprika-spice), dairy products (cheese), drinking contaminated well water and animal contact were associated with human S. Javiana infections. Based on the findings of this study, control of human S. Javiana infection should include three factors, (a) consumption of drinking water after treatment, (b) safe animal contact, and (c) safe food processing and handling procedures. The risk factors of S. Javiana infections identified in the current study provide helpful insight into the major vehicles of transmission of S. Javiana. Eventually, this will help to improve the risk management of this Salmonella serotype to reduce the overall burden of NTS infection in humans.

Influence of temperature, humidity, and diluent type on survival of Salmonella spp. on the surface of raw tomatoes

Tomatoes are an important commodity, placing fourth among most popular vegetables in the U.S. However, fresh tomatoes lack a final pathogen elimination step and have been implicated in Salmonella-related outbreaks. The purpose of the study was to evaluate survival of Salmonella post-drying in three diluents on the surface of green mature tomatoes at 12 °C or 25 °C. Additionally, low and high air relative humidity influence was evaluated at 25 °C on pathogen survival. A five Salmonella rifampin-resistant strain cocktail was double-washed in buffered peptone water (BPW) and resuspended in 0.1% peptone, BPW, or fresh tomato serum. Inoculum (0.1 mL) was allowed to dry on the surface of tomatoes. For study I, tomatoes were placed in 12 °C and 25 °C incubators with no humidity control and sampled on days 0, 1, 3, and 5. For study II, tomatoes were sampled on days 0, 1 (biosafety hood storage) and on day 5 after storage in two 25 oC incubators (low and high relative humidity). Salmonella was recovered from tomatoes (20 mL BPW) and plated (TSA-rif80, 37 °C, 48 hours). Post-drying Salmonella counts (ca. 4.5 – 5.0 log10 CFU.mL-1) remained at 4.03 and 4.40 log10 CFU.mL-1 in serum after 5 days of storage at 12 °C and 25 °C, respectively. Conversely, corresponding counts in BPW and peptone were lower at ca. 1.4 to 1.8 and 2.2 to 2.8 log units at 12 °C and 25 °C, respectively. At low humidity, post-drying Salmonella counts showed highest decline for peptone (final 1.98 log10 CFU.mL-1) compared to BPW (3.79 log10 CFU.mL-1) and tomato serum (4.75 log10 CFU.mL-1) on day 5. Counts declined rapidly to 0.03, 0.56, and 0.44 log10 CFU.mL-1 for peptone, BPW, and tomato serum, respectively, at high humidity on day 5. To summarize, it was shown that increased solutes have protective effect on Salmonella in desiccated conditions, while high humidity storage causes accelerated death of stationary culture within five days storage period.

Microbes associated with fresh produce: Sources, types and methods to reduce spoilage and contamination

Global food security remains one of the most important challenges that needs to be addressed to ensure the increasing demand for food of the fast growing human population is satisfied. Fruits and vegetables comprise an essential component of a healthy balanced diet as they are the major source of both macro- and micronutrients. They are particularly important for communities in developing countries whose nutrition often relies solely on a plant-based diet. Recent advances in agriculture and food processing technologies have facilitated production of fresh, nutritious and safe food for consumers. However, despite the development of sophisticated chemical and physical methods of food and equipment disinfection, fresh-cut produce and fruit juice industry still faces significant economic losses due to microbial spoilage. Furthermore, fresh produce remains an important source of pathogens that have been causing outbreaks of human illness worldwide. This chapter characterizes common spoilage and human pathogenic microorganisms associated with fresh-cut produce and fruit juice products, and discusses the methods and technology that have been developed and utilized over the years to combat them. Substantial attention is given to highlight advantages and disadvantages of using these methods to reduce microbial spoilage and their efficacy to eliminate human pathogenic microbes associated with consumption of fresh-cut produce and fruit juice products.

Survival of Escherichia coli O157:H7 and Salmonella on Bruised and Unbruised Tomatoes from Three Ripeness Stages at Two Temperatures

Tomatoes are one of the major fresh produce commodities consumed in the United States. Harvesting tomato fruit at a later stage of development can enhance consumer acceptance but can also increase damage due to bruising. Bruising can affect the quality of whole tomatoes by causing an unacceptable appearance and accelerating decay. Bruising may also facilitate bacterial attachment to the fruit surface and support growth of pathogens. This study evaluated the survival and/or proliferation of Escherichia coli O157:H7 and Salmonella on the surface of artificially bruised and unbruised tomatoes at three ripeness stages (breaker, pink, and red) and two storage temperatures (10 and 20°C). A total of 1,440 tomatoes, 720 for each organism, were analyzed. Both E. coli O157:H7 and Salmonella counts declined significantly ( P < 0.05) on the bruised and unbruised tomatoes over the 7-day storage period, by approximately 2.5 and 2.0 log, respectively. E. coli O157:H7 was not detected on pink tomatoes on day 7, whereas Salmonella persisted on the tomato surfaces throughout the 7-day study at all ripeness stages. Bruising had no significant effect ( P > 0.05) on the survival of E. coli O157:H7 (CFU per tomato) compared with the unbruised tomatoes, in most cases. Tomatoes from the red ripeness stage showed a significant effect ( P < 0.05) of bruising on Salmonella survival at both 10 and 20°C. Similar to the colony count results, the frequency (presence or absence) of inoculated tomatoes with detectable levels of inoculated bacteria decreased significantly ( P < 0.05) over time. At the lower temperature, E. coli O157:H7 was recovered from significantly higher ( P < 0.05) numbers of breaker and pink tomatoes, whereas there was no effect of temperature on the overall survival of E. coli O157:H7 on red tomatoes. Results from this study are essential for understanding the effects of bruising on produce safety and for producers and packers to develop mitigation strategies to control pathogenic and spoilage organisms.

Behavior of Salmonella Typhimurium on Fresh Strawberries Under Different Storage Temperatures and Wash Treatments

Fresh strawberries are one of the most popular fruits in China and are vulnerable to microbial contamination. In this study, the behavior of Salmonella Typhimurium on fresh strawberries stored at refrigeration and room temperatures, as well as the effectiveness of mild heat wash treatments at 47, 50, and 53°C on bacterial survival was investigated. The modified Gompertz, Huang, log-linear, and Weibull models were used to fit bacterial growth and survival curves under different treatments. A secondary model based on linear regression was developed to describe the effect of washing temperature on the kinetic parameters of S. Typhimurium survival derived from the Weibull model. During 72 h storage, S. Typhimurium on fresh strawberries stored at 4°C was reduced by 1.35 log CFU/g and growth of 5.64 log CFU/g was observed when strawberries were stored at 25°C. Bacterial reductions of 1.22 ± 0.15, 1.92 ± 0.06, 2.27 ± 0.07 log CFU/g were obtained when washing was carried out at 47, 50 and 53°C for 240 s, respectively. The wash temperature was an important parameter for bacterial inactivation and bacterial populations declined significantly in conjunction with washing time (p < 0.05). Warm wash treatments lead the visible color changes of strawberries, showing a slightly darker appearance while acceptable. The goodness-of-fit indices indicated that the log-linear model provided a satisfactory fit to describe the bacterial survival at 4°C. According to the smaller Akaike information criterion (AIC) value, the modified Gompertz model performed slightly better than the Huang model in describing bacterial growth at 25°C. The high adj-R2 (≥0.90) and small RMSE (≤0.22) indicated the Weibull model better described bacterial behavior under mild heat treatments. We found a close linear relationship between wash temperatures and ln k and ln n. These models were validated by independent experimental data and the values of the bias and accuracy factors fell into the acceptable range.

Postharvest Supply Chain with Microbial Travelers: a Farm-to-Retail Microbial Simulation and Visualization Framework

Fresh produce supply chains present variable and diverse conditions that are relevant to food quality and safety because they may favor microbial growth and survival following contamination. This study presents the development of a simulation and visualization framework to model microbial dynamics on fresh produce moving through postharvest supply chain processes. The postharvest supply chain with microbial travelers (PSCMT) tool provides a modular process modeling approach and graphical user interface to visualize microbial populations and evaluate practices specific to any fresh produce supply chain. The resulting modeling tool was validated with empirical data from an observed tomato supply chain from Mexico to the United States, including the packinghouse, distribution center, and supermarket locations, as an illustrative case study. Due to data limitations, a model-fitting exercise was conducted to demonstrate the calibration of model parameter ranges for microbial indicator populations, i.e., mesophilic aerobic microorganisms (quantified by aerobic plate count and here termed APC) and total coliforms (TC). Exploration and analysis of the parameter space refined appropriate parameter ranges and revealed influential parameters for supermarket indicator microorganism levels on tomatoes. Partial rank correlation coefficient analysis determined that APC levels in supermarkets were most influenced by removal due to spray water washing and microbial growth on the tomato surface at postharvest locations, while TC levels were most influenced by growth on the tomato surface at postharvest locations. Overall, this detailed mechanistic dynamic model of microbial behavior is a unique modeling tool that complements empirical data and visualizes how postharvest supply chain practices influence the fate of microbial contamination on fresh produce.IMPORTANCE Preventing the contamination of fresh produce with foodborne pathogens present in the environment during production and postharvest handling is an important food safety goal. Since studying foodborne pathogens in the environment is a complex and costly endeavor, computer simulation models can help to understand and visualize microorganism behavior resulting from supply chain activities. The postharvest supply chain with microbial travelers (PSCMT) model, presented here, provides a unique tool for postharvest supply chain simulations to evaluate microbial contamination. The tool was validated through modeling an observed tomato supply chain. Visualization of dynamic contamination levels from harvest to the supermarket and analysis of the model parameters highlighted critical points where intervention may prevent microbial levels sufficient to cause foodborne illness. The PSCMT model framework and simulation results support ongoing postharvest research and interventions to improve understanding and control of fresh produce contamination.

Attachment of Salmonella enterica on Mangoes and Survival Under Conditions Simulating Commercial Mango Packing House and Importer Facility

Consumption of raw mangoes has led to multiple Salmonella-associated foodborne outbreaks in the United States. Although several studies have investigated the epiphytic fitness of Salmonella on fresh produce, there is sparse information available on the survival of Salmonella on mangoes under commercial handling and storage conditions. Hence, the objective of the study was to evaluate the survival of Salmonella on mangoes under ambient conditions simulating the mango packing house and importer facility. Further, the ability of the pathogen to adhere and attach on to the mango fructoplane was also investigated. For the attachment assays, mango skin sections were inoculated with fifty microliters of S. Newport suspension (6.5 log CFU/skin section) and minimum time required for adhesion and attachment were recorded. With the survival assays, unwaxed mangoes were spot inoculated with the Salmonella cocktail to establish approximately 4 and 6.5 log CFU/mango. The fruits were then subjected to different storage regimens simulating fruit unloading, waxing, and storage at the packing house and ripening and storage at the importer facility. Results of our study reveal that Salmonella was able to adhere on to the fructoplane immediately after contact. Further, formation of attachment structures was seen as early as 2 min following inoculation. With the survival assays, irrespective of the inoculum levels, no significant increase or decrease in pathogen population was observed when fruit were stored either at ambient (29-32°C and RH 85-95%, for 48 h), ripening (20-22°C and RH 90-95% for 9 days) or refrigerated storage (10-15°C and 85-95% for 24-48 h) conditions. Therefore, once contaminated, mangoes could serve as potential vehicles in the transmission of Salmonella along the post-harvest environment. Hence development and adoption of effective food safety measures are warranted to promote the microbiological safety of mangoes.

Salmonella enterica Contamination of Market Fresh Tomatoes: A Review

Salmonella contamination associated with market fresh tomatoes has been problematic for the industry and consumers. A number of outbreaks have occurred, and dollar losses for the industry, including indirect collateral impact to agriculturally connected communities, have run into the hundreds of millions of dollars. This review covers these issues and an array of problems and potential solutions surrounding Salmonella contamination in tomatoes. Some other areas discussed include (i) the use of case-control studies and DNA fingerprinting to identify sources of contamination, (ii) the predilection for contamination based on Salmonella serovar and tomato cultivar, (iii) internalization, survival, and growth of Salmonella in or on tomatoes and the tomato plant, in biofilms, and in niches ancillary to tomato production and processing, (iv) the prevalence of Salmonella in tomatoes, especially in endogenous regions, and potential sources of contamination, and (v) effective and experimental means of decontaminating Salmonella from the surface and stem scar regions of the tomato. Future research should be directed in many of the areas discussed in this review, including determining and eliminating sources of contamination and targeting regions of the country where Salmonella is endemic and contamination is most likely to occur. Agriculturalists, horticulturalists, microbiologists, and epidemiologists may make the largest impact by working together to solve other unanswered questions regarding tomatoes and Salmonella contamination.

Outgraded produce variably retains surface inoculated Escherichia coli through washing

The use of secondary quality produce has gained attention as a solution to food waste in both the U.S. and Europe. The purpose of this study was to evaluate the impact of using secondary quality or outgraded produce on the retention of surface inoculated E. coli following a rinse treatment on four model fresh produce systems (apple, tomato, carrot, lettuce). A three-strain cocktail of rifampicin-resistant generic E. coli, with a concentration of 9.0 log CFU/mL, was spot-inoculated on the intact surfaces of U.S. No.1 grade produce items and damaged or decayed areas of outgraded produce items. Generally, outgraded produce of all four kinds retained higher levels of inoculated E. coli following two postharvest treatments, chlorinated (150 ppm) or water only. However, physical damage, those defects which compromised the integrity of the produce surface, lead to significantly greater E. coli levels following rinsing than did physiological defects. Compared to U.S. No.1 quality apples, outgraded apples retained 4.3 ± 1.4 log CFU/g more E. coli following water only treatment, and 3.6 ± 1.7 log CFU/g more following chlorine treatment. Outgraded tomatoes retained significantly more (3.5 ± 1.1 log CFU/g) inoculated E. coli following water only rinse and 3.0 ± 1.4 log CFU/g more inoculated E. coli following chlorine treatment than U.S. No.1 quality tomatoes did under the same treatment conditions. Outgraded carrots retained 1 ± 1.1 log more CFU/g inoculated E. coli following water only treatment and 0.5 ± 0.8 log more CFU/g inoculated E. coli following chlorine treatment, compared to U.S. No.1 carrots. Outgraded lettuce leaves retained 1.6 ± 0.5 log CFU/g more inoculated E. coli following water only treatment and 4.1 ± 0.4 log CFU/g more inoculated E. coli following chlorine treatment than did U.S. No.1 quality lettuce leaves under the same treatment conditions. Treating with 150 ppm chlorine was not sufficient to eliminate the increased microbial retention associated with secondary quality or outgraded produce, and the efficacy of disinfection was greatly affected by type of defect. Apples with physical damage retained significantly higher E. coli loads than did those with physiological defects, an additional 2.6 log CFU/g under chlorine treatment and 0.8 log CFU/g more under was water only treatment. Tomatoes with physical damage had a 1.3-log CFU/g and 0.6-log CFU/g average increase of retained E. coli counts compared to those with physiological defects following a chlorine and water only treatment, respectively. Although a chlorine dip provided only a modest reduction in pathogens, generally, outgraded produce with physiological defects may present less food safety risks if introduced into the fresh market than does produce with physical damage due to their enhanced retention of bacterial cells. Therefore, as industry considers how to minimize its food waste problem, preferentially directing physically damaged produce away from the fresh market will help to minimize risk while maximizing food resources.

Simultaneous and individual quantitative estimation of Salmonella, Shigella and Listeria monocytogenes on inoculated Roma tomatoes (Lycopersicon esculentum var. Pyriforme) and Serrano peppers (Capsicum annuum) using an MPN technique

Simultaneous and individual enumeration of Salmonella, Shigella and Listeria monocytogenes was compared on inoculated Roma tomatoes and Serrano peppers using an Most Probable Number (MPN) technique. Samples consisting of tomatoes (4 units) or peppers (8 units) were individually inoculated with a cocktail of three strains of Salmonella, Shigella or L. monocytogenes, or by simultaneous inoculation of three strains of each pathogen, at low (1.2-1.7 log CFU/sample) and high (2.2-2.7 log CFU/sample) inocula. Samples were analyzed by an MPN technique using universal pre-enrichment (UP) broth at 35 °C for 24 ± 2 h. The UP tubes from each MPN series were transferred to enrichment and plating media following adequate conventional methods for isolating each pathogen. Data were analyzed using multifactorial analysis of variance (p < 0.05) and LSD multiple rang test. There were differences (p < 0.05) in recovery of simultaneous and individual bacteria inoculated (individual > simultaneous), type of bacteria (Salmonella > Shigella and L. monocytogenes), type of sample (UP broth > pepper and tomato), and inoculum level (high > low). The MPN technique was effective for Salmonella on both commodities. Shigella counts were higher on tomatoes compared to peppers, (p < 0.05), and for L. monocytogenes on peppers (p < 0.05).

Inactivation of Salmonella in grape tomato stem scars by organic acid wash and chitosan-allyl isothiocyanate coating

The objective of this study was to evaluate inactivation of inoculated Salmonella enterica on grape tomato stem scars exploiting integrated treatment of organic acid wash (AW) followed by chitosan-allyl isothiocyanate (CT-AIT) coating. The treatment effect on microbial loads and fruit quality during 21days storage at 10°C was also determined. A bacterial cocktail containing three serotypes of Salmonella enterica was used for this study based on their association with produce-related outbreaks. Tomatoes were spot inoculated on stem scars and then immersed in an organic acid solution (700ml) containing 0.5% (v/v) each of acetic (AA) and formic acid (FA) to wash under mild agitation for 1min at ambient temperature (22°C) followed by 1min dipping in a coating solution containing 6mlAIT/g CT. AW in 0.5% organic acid (AA+FA) for 1min reduced Salmonella population by 2.7logCFU/g from an initial load of 7.8logCFU/g, while additional coating treatment of AW tomatoes reduced the pathogens on stem scars to undetectable levels (<0.7logCFU/g), achieving, in combination, a >7logCFU/g reduction for the pathogen. Although the populations of Salmonella in the controls (approx. 7.8logCFU/g stem scar) did not change significantly during 21days of storage at 10°C, the populations were reduced to undetectable level in the integrated (AW plus CT-AIT) treated stem scars on day 1 and no regrowth was observed during storage. The treatment significantly (p<0.05) reduced background bacterial loads to approx. 1.3logCFU/g and the population remained unchanged through day 21 at 10°C. The treatment also completely inactivated mold and yeast on day 1 with no growth reoccurrence. These results indicate that the integrated treatment can provide a safe and effective intervention strategy for grape tomatoes.

Biofilm of Escherichia coli O157:H7 on cantaloupe surface is resistant to lauroyl arginate ethyl and sodium hypochlorite

Biofilms formed by Escherichia coli O157:H7 on cantaloupe rind were characterized in this study. Cantaloupe rind pieces inoculated with E. coli O157:H7 B6-914 was sampled after 2, 12, and 24h incubation for imaging with cryo-scanning electron microscopy (Cryo-SEM) or treating with lauroyl arginate ethyl (LAE) or sodium hypochlorite (SHC). Cryo-SEM images showed that E. coli O157:H7 formed a biofilm within 12h on the rind surface. For rind samples treated with LAE or SHC, the residual cell counts were significantly different (p<0.05) between 2 and 12h incubation, and between 2 and 24h of incubation. For the 2h incubation samples, E. coli O157:H7 was undetectable (>5-log reduction) after treatment with 2000μg/mL of LAE or SHC. In contrast, for 12h incubation samples, 2000μg/mL of LAE or SHC could only achieve 1.74 or 1.86-log reduction, respectively. The study showed the low efficacy of LAE and SHC on cantaloupe rind surface to reduce the E. coli biofilm, suggesting the needs for cantaloupe cleaning methods beyond washing with conventional antimicrobial agents.

Determination of Optimum Sanitizer Levels for Prevention of Salmonella Cross-Contamination of Mature Round Tomatoes in a Laboratory Model Flume System

Salmonella has been reported to be involved in several foodborne illness outbreaks, many of which resulted from consumption of raw tomatoes. This research aimed to optimize and evaluate the concentration of free chlorine (hypochlorous acid [HOCl]) used as a sanitizer to prevent cross-contamination of tomatoes inoculated with a cocktail of five rifampin-resistant Salmonella enterica serovars in a laboratory-based model flume system. Organic load, prepared using sterilized Scotts Premium Topsoil, was added in different quantities to the flume wash water to simulate real-world packinghouse conditions. In a typical packinghouse operation utilizing a recirculating flume system, the organic matter washed from tomato surfaces accumulates over time. In this study, different concentrations (0, 25, 50, 75, and 100 ppm) of HOCl were used as sanitizers under three organic load conditions (0, 650, and 1,000 mg/L chemical oxygen demand). Results showed that 100 ppm of HOCl was necessary to prevent Salmonella cross-contamination of uninoculated tomatoes in the model flume system in the presence of organic loading. Also, when treated with 100 ppm of HOCl, Salmonella levels were reduced by >4.5 log CFU per tomato from inoculated tomatoes in the presence of organic load. At 75 ppm of HOCl, Salmonella cross-contamination was prevented, but only in the absence of organic loading. In studies in which plate counts were negative, whole tomato enrichment studies were performed. No cross-contamination of uninoculated tomatoes was recorded when 100 ppm of HOCl was used, even in the presence of high organic load (1,000 mg/L chemical oxygen demand). Although sanitizer application reduces contamination on tomato surfaces, the primary function of sanitizers in the wash water is to prevent cross-contamination.

Coliform Contamination of Peri-urban Grown Vegetables and Potential Public Health Risks: Evidence from Kumasi, Ghana

Peri-urban vegetable farming in Ghana is an important livelihood activity for an increasing number of people. However, increasing quality and public health concerns have been raised, partly because freshwater availability for irrigation purposes is a major constraint. This paper investigated on-farm vegetable contamination and potential health risks using samples of lettuce, spring onions and cabbage randomly selected from 18 vegetable farms in peri-urban Kumasi, Ghana. Vegetable samples were tested for total coliform, fecal coliform, Escherichia coli and Salmonella spp. bacteria contamination using the Most Probable Number method. Results show high contamination levels of total and fecal coliforms, and Escherichia coli bacteria in all 18 vegetable samples. The mean total coliform/100 ml concentration for spring onions, lettuce and cabbage were 9.15 × 10(9), 4.7 × 10(7) and 8.3 × 10(7) respectively. The mean fecal coliform concentration for spring onions, lettuce and cabbage were also 1.5 × 10(8), 4.15 × 10(7) and 2.15 × 10(7) respectively, while the mean Escherichia coli bacteria contamination for spring onions, lettuce and cabbage were 1.4 × 10(8), 2.2 × 10(7) and 3.2 × 10(7) respectively. The level of total coliform, fecal coliform and Escherichia coli bacteria contamination in all the vegetable samples however declined as the distance between the main water source (Wiwi River) and farms increases. Nonetheless, all contamination levels were well above acceptable standards, and could therefore pose serious public health risks to consumers. Increased education and supervision of farmers, as well as public health and food hygiene education of consumers, are critical to reducing on-farm vegetable contamination and the health risks associated with consumption of such vegetables.

Inactivation of Listeria monocytogenes, Salmonella spp. and Escherichia coli O157:H7 on cantaloupes by octenidine dihydrochloride

The efficacy of a new generation disinfectant, octenidine dihydrochloride (OH), as wash and coating treatments for reducing Listeria monocytogenes (LM), Salmonella spp. (SAL), and Escherichia coli O157:H7 (EC) on cantaloupe was investigated. Cantaloupe rind plugs inoculated separately with the three bacterial species (∼8 log CFU/cm(2)) were washed for 1, 3, 5 min at 25 °C in water, or chlorine (200 ppm), ethanol (1%), OH (0.01, 0.05, 0.1%) and surviving populations were measured after treatment. Additionally, inoculated cantaloupe rind plugs were coated with 2% chitosan or chitosan containing OH (0.01, 0.05, 0.1%) and sampled for surviving pathogens. Subsequently, the antimicrobial efficacy of OH wash and coating (0.1, 0.2%) on whole cantaloupes was determined. All OH wash reduced LM, SAL, and EC on cantaloupe rinds by > 5 log CFU/cm(2) by 2 min, and reduced populations to undetectable levels (below 2 log CFU/cm(2)) by 5 min (P < 0.05). Similarly, OH coating on cantaloupe rinds reduced the pathogens by 3-5 log /cm(2) (P < 0.05). Washing and coating whole cantaloupes with OH reduced the three pathogens by at least 5 log and 2 log CFU/cm(2), respectively (P < 0.05). Results suggest that OH could be used as antimicrobial wash and coating to reduce LM, SAL, and EC on cantaloupes.

Control of Salmonella Cross-Contamination between Green Round Tomatoes in a Model Flume System

Tomato Best Management Practices require Florida packers to treat tomatoes in a flume system containing at least 150 ppm of free chlorine or other approved sanitizer. However, research is needed to determine the ability of these sanitizers to prevent the transfer of pathogens from contaminated to uncontaminated tomatoes, particularly under realistic packinghouse conditions. The goal of this research was to assess the minimum levels of sanitizer needed to prevent Salmonella cross-contamination between tomatoes in a model flume system under clean conditions and conditions where organic matter was added. Inoculated tomatoes (ca. 8.3 log CFU per tomato) were treated along with uninoculated tomatoes in a model flume system containing 0, 10, or 25 ppm of hypochlorous acid (HOCl) under organic loading conditions of 0, 500, or 4,000 ppm of chemical oxygen demand (COD). In the absence of HOCl, uninoculated tomatoes were highly contaminated (ca. 5 log CFU per tomato) by 15 s. No contamination was detectable (<2 log CFU per tomato) on uninoculated tomatoes when HOCl was present, except with 10 ppm at 4,000 ppm of COD, suggesting failure of 10 ppm of HOCl as a sanitizer under very high organic loading conditions. In the presence of HOCl or peroxyacetic acid, Salmonella was undetectable (<1 log CFU/ml) in the model flume water samples after 2 and 30 s, respectively. Upon enrichment, none of the uninoculated tomatoes treated with 25 ppm of HOCl for 120 s were positive for Salmonella, even in the presence of organic loading at 500 ppm of COD. Based on these findings, 25 ppm of HOCl may be adequate to prevent cross-contamination when the concentration is properly maintained, COD does not exceed 500 ppm, and tomatoes are treated for at least 120 s. Further validation in a larger commercial setting and using higher organic loading levels is necessary because managing HOCl at this low concentration is difficult, especially in a recirculating system. The use of less sanitizer by packers could reduce chemical and disposal costs.

Quantitative proteomic analysis of the Salmonella-lettuce interaction

Human pathogens can internalize food crops through root and surface uptake and persist inside crop plants. The goal of the study was to elucidate the global modulation of bacteria and plant protein expression after Salmonella internalizes lettuce. A quantitative proteomic approach was used to analyse the protein expression of Salmonella enterica serovar Infantis and lettuce cultivar Green Salad Bowl 24 h after infiltrating S. Infantis into lettuce leaves. Among the 50 differentially expressed proteins identified by comparing internalized S. Infantis against S. Infantis grown in Luria Broth, proteins involved in glycolysis were down-regulated, while one protein involved in ascorbate uptake was up-regulated. Stress response proteins, especially antioxidant proteins, were up-regulated. The modulation in protein expression suggested that internalized S. Infantis might utilize ascorbate as a carbon source and require multiple stress response proteins to cope with stresses encountered in plants. On the other hand, among the 20 differentially expressed lettuce proteins, proteins involved in defense response to bacteria were up-regulated. Moreover, the secreted effector PipB2 of S. Infantis and R proteins of lettuce were induced after bacterial internalization into lettuce leaves, indicating human pathogen S. Infantis triggered the defense mechanisms of lettuce, which normally responds to plant pathogens.

Isolation and characterization of plant and human pathogenic bacteria from green pepper (Capsicum annum L.) in Riyadh, Saudi Arabia

Forty-three bacterial isolates in five genera were recovered from naturally infected green pepper fruits (38 samples) showing dark brown, irregular-shaped splotches. The pathogenicity test was performed on healthy green pepper fruits and red colonies were from inoculated fruits showing the same symptoms and the infected area developed into soft rot. Their identification was based on phenotypic characters and sequence of the gene fragment coding 16S rRNA. Of 43 isolates, 10 showing splotches on green pepper fruits belonged to genus Serratia on the basis of phenotypic characters. One representative isolate of the genus Serratia has been identified by partial 16S rRNA gene sequencing and phylogenetic analysis as belonging to the Serratia rubidaea and has the potential to cause spot on green pepper. Eleven phytopathogenic bacterial isolates were also obtained at the same time but did not induce any splotch symptoms on artificially infected green pepper. Five out of 11 bacterial isolates were identified as Ralstonia on the basis of biochemical tests. Partial sequencing of 16S ribosomal gene of representative isolate revealed that the isolate is Ralstonia solanacearum. The six remaining isolates were related to Xanthomonas vesicatoria on the basis of biochemical tests. Twenty-two of opportunistic human pathogens were isolated at the same time and related to Proteus and Klebsiella. Opportunistic human pathogens did not produce any symptoms on artificially infected green pepper. One representative isolate for each genus was identified as Klebsiella oxytoca and Proteus mirabilis based on their partial 16S rRNA gene sequences. The virulence of the S. rubidaea, the causal agent of green pepper fruits splotches was attributed to the production and secretion of a large variety of enzymes capable of degrading the complex polysaccharides of the plant cell wall and membrane constituents.

Fate of Escherichia coli O157:H7 and Salmonella on whole strawberries and blueberries of two maturities under different storage conditions

Strawberries and blueberries harvested at or near full-ripe maturity tend to be less firm and more susceptible to bruising during harvest and transport. The objective of this research was to determine the fate of Escherichia coli O157:H7 and Salmonella on bruised and intact surfaces of whole strawberries and blueberries at shipping (2°C) and retail display (15.5°C) temperatures. Strawberries and blueberries were either purchased from a supermarket or were harvested immediately prior to use; they were bruised using established protocols, were spot inoculated, and were incubated at 2 and 15.5°C. Strawberries, subjected to modified atmospheres, were further transferred to bags and were sealed in with an initial atmosphere of ca. 10% CO2 and 5% O2. Strawberries were sampled at 0, 2, 5, and 24 h and on days 3 and 7; blueberries were sampled on days 0, 1, 3, and 7. After stomaching, samples were enumerated on nonselective and selective media, and populations were recorded as log CFU per berry. At both storage temperatures, population declines for both E. coli O157:H7 and Salmonella were seen under all conditions for strawberries. At 2 ± 2°C, E. coli O157:H7 and Salmonella populations on blueberries declined over 7 days under all conditions. At 15.5 ± 2°C, E. coli O157:H7 populations declined; however, Salmonella populations initially declined but increased to populations near or above initial populations over 7 days on blueberries. No overall significant differences were observed between bruised and intact treatments or between the two maturity levels for strawberries and blueberries. Modified atmospheric conditions did not affect the behavior of E. coli O157:H7 and Salmonella on strawberries at both temperatures. This research indicates that E. coli O157:H7 and Salmonella do not grow on strawberries at shipping or retail display temperatures, even when they are harvested at a maturity prone to bruising; however, Salmonella growth may occur on bruised full ripe blueberries under retail display temperatures.

Effects of postharvest handling conditions on internalization and growth of Salmonella enterica in tomatoes

Salmonella internalization in tomatoes during postharvest handling is a major food safety concern. This study was conducted to determine the effect of immersion time, immersion depth, and temperature differential between bacterial suspension and tomato pulp on the internalization of Salmonella enterica in tomato fruits. The effect of storage temperature and duration on the survival and growth of internalized Salmonella cells was also evaluated. Overall, immersion time significantly affected the incidence and extent of S. enterica internalization (P < 0.0001), with a linear correlation between immersion time and Salmonella internalization. The depth of Salmonella internalization in tomato tissues also increased with increasing immersion time. Immersion time also significantly influenced the degree to which the temperature differential affected Salmonella internalization. With an immersion time of 2 min, the temperature differential had no significant effect on Salmonella internalization (P = 0.2536). However, with an immersion time of 15 min, a significantly larger Salmonella population became internalized in tomatoes immersed in solutions with a -30°F (-16.7°C) temperature differential. Internalized S. enterica cells persisted in the core tissues during 14 days of storage. Strain type and storage duration significantly affected (P < 0.05) both the frequency detected and the population of internalized Salmonella recovered, but storage temperatures of 55 to 70°F (12.8 to 21.1°C) did not (P > 0.05). These findings indicate the importance of preventing pathogen internalization during postharvest handling.

Inactivation of Listeria innocua, Salmonella Typhimurium, and Escherichia coli O157:H7 on surface and stem scar areas of tomatoes using in-package ozonation

A novel in-package ozonation device was evaluated for its efficacy in inactivating three microorganisms (viz., Listeria innocua, attenuated Salmonella Typhimurium, and Escherichia coli O157:H7) on tomatoes and for its effect on fruit quality. The device produced ozone inside sealed film bags, reaching a concentration of 1,000 ppm within 1 min of activation. The three bacterial cultures were inoculated onto either the smooth surface or the stem scar areas of the tomatoes, which were then sealed in plastic film bags and subjected to in-package ozonation. L. innocua on tomatoes was reduced to nondetectable levels within 40 s of treatment on the tomato surface, with inactivation of ca. 4 log CFU per fruit on the stem scar area. An increase in treatment time did not result in a proportional increase in bacterial reduction. For E. coli O157:H7 and Salmonella, there was little difference (<1 log) in the effectiveness of the system when comparing surface and scar-inoculated bacteria. Both bacteria were typically reduced by 2 to 3 log CFU per fruit after 2- to 3-min treatments. No negative effects on fruit color or texture were observed during a 22-day posttreatment storage study of ozone-treated tomatoes. These results suggest that the three bacteria responded differently to ozonation and that in-package ozonation may provide an alternative to chemical sanitizers commonly used by the industry.

Inactivation of salmonella on tomato stem scars by edible chitosan and organic Acid coatings

This study was conducted to investigate the efficacy of antimicrobial coatings for inactivation of Salmonella on the surface of tomato stem scars. Scars were inoculated with a four-strain cocktail of Salmonella (serovars Montevideo, Newport, Saintpaul, and Typhimurium) and coated with acid-chitosan solutions. The chitosan coating with three acids (3A plus chitosan), the chitosan coating with one acid, and the three-acid solution without chitosan reduced the populations of Salmonella by 6.0, 3.6, and 5.3 log CFU per stem scar, respectively. Addition of allyl isothiocyanate (10 μl/ml) to the 3A plus chitosan coating did not significantly increase (P > 0.05) the antimicrobial efficacy. Although the populations of Salmonella in the controls (ca. 7.5 log CFU per stem scar) did not change significantly throughout the 14-day storage period at 10° C, Salmonella cells were reduced to undetectable levels (< 0.7 log CFU per stem scar) in the samples treated with 3A plus chitosan coating after two days of storage, and no growth was observed for the remaining storage period. Results from this study demonstrate that coatings of acid plus chitosan provide an alternative antimicrobial intervention for decontamination of tomatoes.

Inactivation of Salmonella on whole cantaloupe by application of an antimicrobial coating containing chitosan and allyl isothiocyanate

This study investigated the antimicrobial effect of a chitosan coating+allyl isothiocyanate (AIT) and nisin against Salmonella on whole fresh cantaloupes. Cantaloupes were inoculated with a cocktail of three Salmonella strains and treated with chitosan, chitosan+AIT, chitosan+nisin, and chitosan+AIT+nisin coatings. With AIT concentrations increasing from 10 to 60 μl/ml, the antibacterial effects of coating treatments against Salmonella increased. Chitosan coatings with 60 μl/ml AIT (chitosan+60AIT) reduced more than 5 log₁₀ CFU/cm² of Salmonella. The addition of nisin to the chitosan-AIT coating synergistically increased the antibacterial effect; coatings with nisin (25 mg/ml or 25,000 IU/ml)+30 μl/ml AIT resulted in a 4.8 log₁₀ reduction of Salmonella. The chitosan+60AIT coating significantly (p<0.05) reduced populations of native bacteria on cantaloupes to ca. 2 log₁₀ CFU/cm² during the first 6 days and populations remained unchanged through day 14 at 10 °C. The same coating treatment completely inactivated mold and yeast on cantaloupe at day 1 and no regrowth occurred even up to 14 days of storage. Scanning electron microscopy revealed that cell membrane damage and leakage of intercellular components occurred as a result of the chitosan-AIT coating treatments. No visual changes in overall appearance and color of cantaloupe rind and flesh due to coating treatments were observed. These results indicate that the application of an antimicrobial coating may be an effective method for decontamination of cantaloupes.